feat: export images using Transfer api

Signed-off-by: Jian Zeng <anonymousknight96@gmail.com>

pkg/transfer/local/export.go

@@ -0,0 +1,49 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package local
+
+import (
+	"context"
+
+	"github.com/containerd/containerd/pkg/transfer"
+)
+
+func (ts *localTransferService) exportStream(ctx context.Context, is transfer.ImageExporter, tops *transfer.Config) error {
+	ctx, done, err := ts.withLease(ctx)
+	if err != nil {
+		return err
+	}
+	defer done(ctx)
+
+	if tops.Progress != nil {
+		tops.Progress(transfer.Progress{
+			Event: "Exporting",
+		})
+	}
+
+	err = is.Export(ctx, ts.images, ts.content)
+	if err != nil {
+		return err
+	}
+
+	if tops.Progress != nil {
+		tops.Progress(transfer.Progress{
+			Event: "Completed export",
+		})
+	}
+	return nil
+}

pkg/transfer/local/transfer.go

@@ -72,6 +72,8 @@ func (ts *localTransferService) Transfer(ctx context.Context, src interface{}, d
 		switch d := dest.(type) {
 		case transfer.ImagePusher:
 			return ts.push(ctx, s, d, topts)
+		case transfer.ImageExporter:
+			return ts.exportStream(ctx, d, topts)
 		}
 	case transfer.ImageImporter:
 		switch d := dest.(type) {